Subtitles and Transcript

Gian Giudice

0:12
So last year, on the Fourth of July,experiments at the Large Hadron Colliderdiscovered the Higgs boson.It was a historical day.There's no doubt that from now on,the Fourth of July will be rememberednot as the day of the Declaration of Independence,but as the day of the discovery of the Higgs boson.Well, at least, here at CERN.

0:34
But for me, the biggest surprise of that daywas that there was no big surprise.In the eye of a theoretical physicist,the Higgs boson is a clever explanationof how some elementary particles gain mass,but it seems a fairly unsatisfactoryand incomplete solution.Too many questions are left unanswered.The Higgs boson does not share the beauty,the symmetry, the elegance,of the rest of the elementary particle world.For this reason, the majority of theoretical physicistsbelieve that the Higgs boson could notbe the full story.We were expecting new particles and new phenomenaaccompanying the Higgs boson.Instead, so far, the measurementscoming from the LHC show no signs of new particlesor unexpected phenomena.

1:26
Of course, the verdict is not definitive.In 2015, the LHC will almost doublethe energy of the colliding protons,and these more powerful collisionswill allow us to explore further the particle world,and we will certainly learn much more.

1:47
But for the moment, since we have foundno evidence for new phenomena, let us supposethat the particles that we know today,including the Higgs boson,are the only elementary particles in nature,even at energies much largerthan what we have explored so far.Let's see where this hypothesis is going to lead us.We will find a surprising and intriguing resultabout our universe, and to explain my point,let me first tell you what the Higgs is about,and to do so, we have to go backto one tenth of a billionth of a secondafter the Big Bang.And according to the Higgs theory,at that instant, a dramatic event took placein the universe.Space-time underwent a phase transition.It was something very similar to the phase transitionthat occurs when water turns into icebelow zero degrees.But in our case, the phase transitionis not a change in the way the moleculesare arranged inside the material,but is about a changeof the very fabric of space-time.

3:05
During this phase transition, empty spacebecame filled with a substancethat we now call Higgs field.And this substance may seem invisible to us,but it has a physical reality.It surrounds us all the time,just like the air we breathe in this room.And some elementary particles interactwith this substance, gaining energy in the process.And this intrinsic energy is what we callthe mass of a particle,and by discovering the Higgs boson, the LHChas conclusively proved that this substance is real,because it is the stuff the Higgs bosons are made of.And this, in a nutshell, is the essence of the Higgs story.

3:51
But this story is far more interesting than that.By studying the Higgs theory,theoretical physicists discovered,not through an experimentbut with the power of mathematics,that the Higgs field does not necessarily existonly in the form that we observe today.Just like matter can exist as liquid or solid,so the Higgs field, the substance that fills all space-time,could exist in two states.Besides the known Higgs state,there could be a second state in which the Higgs fieldis billions and billions times denserthan what we observe today,and the mere existence of another stateof the Higgs field poses a potential problem.This is because, according to the lawsof quantum mechanics, it is possibleto have transitions between two states,even in the presence of an energy barrierseparating the two states,and the phenomenon is called,quite appropriately, quantum tunneling.Because of quantum tunneling,I could disappear from this roomand reappear in the next room,practically penetrating the wall.But don't expect me to actually perform the trickin front of your eyes, because the probabilityfor me to penetrate the wall is ridiculously small.You would have to wait a really long timebefore it happens, but believe me,quantum tunneling is a real phenomenon,and it has been observed in many systems.For instance, the tunnel diode,a component used in electronics,works thanks to the wondersof quantum tunneling.

5:45
But let's go back to the Higgs field.If the ultra-dense Higgs state existed,then, because of quantum tunneling,a bubble of this state could suddenly appearin a certain place of the universe at a certain time,and it is analogous to what happens when you boil water.Bubbles of vapor form inside the water,then they expand, turning liquid into gas.In the same way, a bubble of the ultra-dense Higgs statecould come into existence because of quantum tunneling.The bubble would then expand at the speed of light,invading all space, and turning the Higgs fieldfrom the familiar state into a new state.

6:30
Is this a problem? Yes, it's a big a problem.We may not realize it in ordinary life,but the intensity of the Higgs field is criticalfor the structure of matter.If the Higgs field were only a few times more intense,we would see atoms shrinking, neutrons decayinginside atomic nuclei, nuclei disintegrating,and hydrogen would bethe only possible chemical element in the universe.And the Higgs field, in the ultra-dense Higgs state,is not just a few times more intense than today,but billions of times,and if space-time were filled by this Higgs state,all atomic matter would collapse.No molecular structures would be possible, no life.

7:21
So, I wonder, is it possiblethat in the future, the Higgs fieldwill undergo a phase transition and,through quantum tunneling, will be transformedinto this nasty, ultra-dense state?In other words, I ask myself, what is the fateof the Higgs field in our universe?And the crucial ingredient necessaryto answer this question is the Higgs boson mass.And experiments at the LHC found that the massof the Higgs boson is about 126 GeV.This is tiny when expressed in familiar units,because it's equal to something like10 to the minus 22 grams,but it is large in particle physics units,because it is equal to the weightof an entire moleculeof a DNA constituent.

8:16
So armed with this information from the LHC,together with some colleagues here at CERN,we computed the probabilitythat our universe could quantum tunnelinto the ultra-dense Higgs state,and we found a very intriguing result.Our calculations showedthat the measured value of the Higgs boson massis very special.It has just the right valueto keep the universe hangingin an unstable situation.The Higgs field is in a wobbly configurationthat has lasted so farbut that will eventually collapse.So according to these calculations,we are like camperswho accidentally set their tentat the edge of a cliff.And eventually, the Higgs fieldwill undergo a phase transitionand matter will collapse into itself.

9:13
So is this how humanity is going to disappear?I don't think so.Our calculation shows that quantum tunnelingof the Higgs field is not likely to occurin the next 10 to the 100 years,and this is a very long time.It's even longer thanthe time it takes for Italy to form a stable government.

9:37
(Laughter)

9:39
Even so, we will be long gone by then.In about five billion years,our sun will become a red giant,as large as the Earth's orbit,and our Earth will be kaput,and in a thousand billion years,if dark energy keeps on fuelingspace expansion at the present rate,you will not even be able to see as far as your toes,because everything around youexpands at a rate faster than the speed of light.So it is really unlikelythat we will be around to see the Higgs field collapse.

10:17
But the reason why I am interestedin the transition of the Higgs fieldis because I want to address the question,why is the Higgs boson mass so special?Why is it just right to keep the universeat the edge of a phase transition?Theoretical physicists always ask "why" questions.More than how a phenomenon works,theoretical physicists are always interested inwhy a phenomenon works in the way it works.We think that this these "why" questionscan give us cluesabout the fundamental principles of nature.And indeed, a possible answer to my questionopens up new universes, literally.It has been speculated that our universeis only a bubble in a soapy multiversemade out of a multitude of bubbles,and each bubble is a different universewith different fundamental constantsand different physical laws.And in this context, you can only talk aboutthe probability of finding a certain value of the Higgs mass.Then the key to the mysterycould lie in the statistical propertiesof the multiverse.It would be something like what happenswith sand dunes on a beach.In principle, you could imagine to find sand dunesof any slope angle in a beach,and yet, the slope angles of sand dunesare typically around 30, 35 degrees.And the reason is simple:because wind builds up the sand, gravity makes it fall.As a result, the vast majority of sand duneshave slope angles around the critical value,near to collapse.And something similar could happenfor the Higgs boson mass in the multiverse.In the majority of bubble universes,the Higgs mass could be around the critical value,near to a cosmic collapse of the Higgs field,because of two competing effects,just as in the case of sand.

12:31
My story does not have an end,because we still don't know the end of the story.This is science in progress,and to solve the mystery, we need more data,and hopefully, the LHC will soon add new cluesto this story.Just one number, the Higgs boson mass,and yet, out of this number we learn so much.I started from a hypothesis, that the known particlesare all there is in the universe,even beyond the domain explored so far.From this, we discovered that the Higgs fieldthat permeates space-time may be standingon a knife edge, ready for cosmic collapse,and we discovered that this may be a hintthat our universe is only a grain of sandin a giant beach, the multiverse.

13:32
But I don't know if my hypothesis is right.That's how physics works: A single measurementcan put us on the road to a new understandingof the universeor it can send us down a blind alley.But whichever it turns out to be,there is one thing I'm sure of:The journey will be full of surprises.